This invention relates to bearings and bearing mountings disposed between two relatively rotatable structures and in particular to the mounting of metal bearings between rotatable structures of material having a low coefficient of expansion, such as graphite/epoxy.
Antifriction bearing are often mounted by pressing the outer race into a bored housing with a slight interference fit, or by pressing the inner race onto a shaft, again with a slight interference fit. When metal bearings are used with metal housings or metal shafts, this procedure is satisfactory because metals usually have similar coefficients or expansion.
Some materials, however, have no significant coefficient or expansion--graphite/epoxy is one such material. Using such a material with a metallic bearing introduces a problem of differential thermal expansion (or contraction) between the bearing and the housing (the element into which the outer race is pressed) and between the bearing and the "shaft" (the element onto which the inner race is pressed). As an example of such a problem, a metal bearing race expanding due to temperature increase inside of a graphite/epoxy housing, which does not expand, may rupture the housing. If the temperature is lowered, below that which existed when the bearing was pressed into the housing, the bearing race will contract while the housing will not. The bearing race, in this instance, may become so much smaller that it will be only a loose fit (instead of a press fit) in the housing, and may, in fact, fall out of the housing.
The amount of thermal expansion, or contraction, of the metal bearing depends upon the coefficient or thermal expansion of the metallic material and upon the product of the diameter of the bearing in inches and the change in temperature of the bearing, usually measured in degrees fahrenheit. Thus, a large diameter bearing may suffer an unacceptable change in diameter when subjected to a small temperature change; a small diameter bearing may suffer an equally unacceptable diameter change if the change in temperature is sufficiently great. Each bearing application will experience unacceptable dimension or change at a certain predictable combination of bearing diameter and temperature change.
Thus, there is a need for a bearing assembly by which radial motion of the expanding and contracting bearings is allowed without radial constraint or loosening yet the axis of the bearings and the axis of the graphite/epoxy rotatable structures, in which the bearing assemblies are mounted, are maintained in alignment.
Accordingly, it is a principle object of this invention to provide bearings in relatively rotatable structures, of low coefficient of expansion material, which will maintain alignment during temperature changes.
Still another object of this invention is to provide a means for mounting steel bearings in rotating structures of low coefficient of expansion material, such as graphite/epoxy, which allows radial expansion and contraction without constraint or loosening and maintains the alignment of the axis of the bearing with the axes of the rotating elements.